1. Field of the Invention
The present invention relates to a method and an apparatus for driving an edge emission type line head. More particularly, the invention relates to a method and an apparatus for driving an edge emission type line head comprising edge emission type EL (electroluminescent) elements to which positive and negative high voltages are applied alternately.
2. Description of the Related Art
The line printer is one of recently developed printers that operate on the principle of electrophotography. The line printer comprises a photosensitive drum whose periphery is surrounded by a charging device, a line head, a developing device and a transfer device. The peripheral surface of the photosensitive drum is charged at a predetermined potential level by the charging device. When a printing data is supplied to the printer, an optical signal corresponding to that printing data is emitted by light-emitting devices of the line head to form an electrostatic latent image on the charged peripheral surface of the photosensitive drum. The electrostatic latent image on the photosensitive drum is developed by the developing device using toner. The toner-developed image is transferred by transfer device onto an image forming medium such as a sheet of paper. With this type of line printer, edge emission type EL elements may be utilized as a line head.
As shown in FIG. 6, an edge emission type EL element 6 comprises dielectric layers 2 and 3 that sandwich a thin film active layer 1 comprising active elements including zinc sulfide, the surfaces of the layers 2 and 3 having flat electrodes 4 and 5 deposited thereon. In operation, a voltage is applied to the flat electrodes 4 and 5. This causes an edge of the active layer 1 to emit a light beam. A number of such edge emission type EL elements 6 are arranged linearly by thin film technique or the like on a substrate 7, as depicted in FIG. 7. A rod lens array, not shown, is positioned opposite to the tips of the edge emission type EL elements to form a line head 8.
FIG. 8 is a circuit diagram of a typical prior art circuit for driving the line head 8. The edge emission type EL elements 6 of the line head 8 are grouped into blocks composed of n (e.g., 6) continuous devices each. Each block has its channel electrode connected to a channel driver 9. There is provided a common driver 10 comprising m (e.g., 6) common terminals 10.sub.1 through 10.sub.6. Each of the common terminals 10.sub.1 through 10.sub.6 is connected to a connection line 11. The common electrode of the edge emission type EL elements within each block is connected to a different connection line 11, whereby a matrix circuit 12 is formed. The channel drivers 9 are connected to a channel driver control unit 13 and a data control unit 14. The common driver 10 is connected to a common driver control unit 15.
The operation of the line head 8 will now be described with reference to the timing chart of FIGS. 9-c. The channel electrode of the edge emission type EL elements 6 is fed with channel electrode voltages Va and Vb from the channel drivers 9. The common electrode of the edge emission type EL elements 6 is supplied with common electrode voltages V.sub.P and V.sub.N from the common terminals 10.sub.1 through 10.sub.6 of the common driver 10. "V.sub.P " stands for a positive high voltage and "V.sub.N " for a negative high voltage. When the difference between the voltages Va and Vb from the channel driver 9 on the one hand, and the corresponding high voltages V.sub.P and V.sub.N from the common driver 10 on the other is greater than a threshold voltage V.sub.th, the edge emission type EL element 6 emits light. According to electrophotography, the emitted light reaches a photosensitive body to form an electrostatic latent image thereon. The electrostatic latent image is then developed and transferred onto an appropriate sheet of paper.
If it is desired to perform printing on a letter size sheet of a density of 300 dpi (dot per inch), it is necessary to arrange about 2,500 edge emission type EL elements 6 in a row. To drive such numerous EL elements 6 would require very complicated driving circuitry. This difficulty is overcome using the matrix circuit 12 mentioned above. In operation, the common electrode voltages V.sub.P and V.sub.N, are output successively from the common terminals 10. In synchronism with the voltages V.sub.P and V.sub.N, the channel electrode voltages Va and Vb are fed from the channel electrodes. For example, it may be desired first to activate the edge emission type EL element 6 connected to the common terminal 10.sub.1 and to the channel electrode of the n-th block. In such a case, the channel electrode voltages Va and Vb needed to activate the edge emission type EL element 6 are output from the channel electrode of the n-th block in synchronism with the common electrode voltages V.sub.P and V.sub.N being output from the common terminal 10.sub.1. Next, it may be desired to activate the edge emission type EL element 6 connected to the common terminal 10.sub.2 and to the channel electrode of the n-th block. This requires getting the channel electrode of the n-th block to output the channel electrode voltages Va and Vb needed to activate the EL element 6, the output being performed in synchronism with the common electrode voltages V.sub.P and V.sub.N output from the common terminal 10.sub.2 following the voltage output from the common terminal 10.sub.1. In like manner, the edge emission type EL elements 6 connected to the common terminals 10.sub.1 through 10.sub.6 have been activated. This completes the image forming operation for one line.
Aside from detailed analyses, experiments have revealed that applying only positive (or negative) high voltages to the common electrode of the edge emission type EL elements shortens the service life of the latter. This flaw is circumvented traditionally by having the common driver 10 output alternately the positive voltage V.sub.P and the negative voltage V.sub.N. One way of implementing the alternate positive-negative voltage output scheme is to output successively pairs of positive and negative pulses to m (e.g., 6) common terminals, as illustrated in FIGS. 10a-f. FIGS. 11a-f shows another way of implementing the same scheme, whereby the common terminals output positive and negative pulses in a successively alternate manner and then reverse the polarities of the respective pulses to output them likewise. In FIGS. 10a-f and 11a-f, the output from the first common terminal 10.sub.1 is denoted as Common (1), the output from the second common terminal 10.sub.2 as Common (2), the output from the third common terminal 10.sub.3 as Common (3), and so on. The output patterns shown in FIGS. 10 and 11 are each the unit in which a one-line image is formed.
A major disadvantage of the above-described prior art is as follows: during image formation, something can happen to halt the image forming function halfway. For example, the printer cover may be opened inadvertently during image formation. The cover action illustratively operates a cover-open switch to bring the image forming function to a stop. The image forming function may also be halted in case of a jammed sheet. The moment a sheet is found to be jammed, the image forming operation is stopped, and the output from the common driver 10 also ends simultaneously. Suppose that while the line head 8 is being driven under the driving scheme of FIG. 10, something happens to stop the image forming function (cover-open switch operated, jammed sheet detected, etc.). At that point, the output from the common driver 10 is also stopped. FIGS. 12a-c shows a state in which something happened to stop the image forming function after the second common terminal 10.sub.2 output a positive high voltage; FIGS. 12f-g ) shows a state in which something happened to stop the image forming function while a positive high voltage is being output from the second common terminal 10.sub.2. In either case, no further output comes out of the common driver 10.
When the line head 8 is driven under the driving scheme of FIGS. 11a-f, the output of the common driver 10 is stopped the moment something happens to halt the image forming function. FIGS. 13a-g shows a state in which something happened to stop the image forming function after the third common terminal 10.sub.3 outputs a negative high voltage. In this case, no further output is made by the common driver 10 while the output of one-line pulses is suspended.
In other words, the states shown in FIGS. 12a-j prevent one edge emission type EL element 6 from getting fed with a pair of a positive and a negative high voltage. This, as described above, leads to shortening the service life of the edge emission type EL element 6. The same holds true for the state of FIGS. 13a-g. When the emission frequency varies from one edge emission type EL element 6 to another in a single line, the individual devices 6 can incur differences in the duration of their service life. As a result, the service life of the line head 8 can be shortened inordinately.